This invention relates to a nitrocellulose composition suitable for compounding in polyurethane paint and a process for production thereof.
As is known in the art, nitrocellulose is often introduced or compounded in polyurethane paint, utilizing the reactivity of isocyanate to accelerate the hardening of a coated film and to improve the dispersibility of pigments and the smoothness of the coated film.
Since nitrocellulose has the possibility of spontaneously catching fire when allowed to stand in a dry state, a wetting agent is usually added to nitrocelluloses on the market, for the purpose of reducing the danger of fire during the storage, transportation, and handling thereof. As such wetting agents, alcohols such as methanol, ethanol, isopropanol, butanol, etc. are generally used. The wetting of nitrocellulose is carried out by washing water wetted-nitrocellulose with the wetting agent, but it is difficult to completely replace the water by the wetting agent, and water inevitably remains in the nitrocellulose. Nitrocelluloses on the market, therefore, ordinarily contain about 30% by weight alcohol and from 2 to 3% by weight water. (Hereinafter, all parts, ratios and percents are by weight unless otherwise indicated.)
In adding these nitrocelluloses to polyurethane paints, a problem arises in that the alcohol and water in the nitrocellulose undergo reaction with isocyanate groups, a hardening component of the polyurethane paint. The reaction of the isocyanate with the alcohol and water leads to a reduction in the crosslinking reactions necessary for hardening of a coated film and to the formation of low molecular weight compounds which act as plasticizers. This results in the reduction in the strength of a coated film. It is, therefore, preferred that nitrocelluloses to be added to polyurethane paints do not contain compounds which are reactive with isocyanate, particularly alcohols and water, which yield low molecular weight compounds through the reaction with isocyanate.
In addition to these alcohols, resins and plasticizers are often used as wetting agents for use in nitrocelluloses. For example, "Clear Chip" (trade name for a product of Asahi Kasei Kogyo Kabushiki Kaisha) contains dibutyl phthalate as a wetting agent, and in Japanese Patent Application (OPI) No. 142756/1977 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), a vinyl chloride based resin is used as a wetting agent. In these cases, the above described problems have been solved by using those wetting agents which do not react with isocyanate. U.S. Pat. No. 3,763,061 discloses a method of positively solving the problems by employing as wetting agents isocyanate prepolymers containing isocyanate groups.
Nitrocelluloses, however, containing the above described wetting agents which remain in a coated film or participate in the formation of a coated film necessarily exert great influences on the physical properties of the coated film. Particularly, these nitrocelluloses impose limitations on the usefulness of the coated film with respect to the physical properties thereof, and their use in polyurethane paints having a wide variety of applications is limited. Therefore, nitrocelluloses which are to be used in polyurethane paints should desirably contain no components remaining in a coated film other than the nitrocellulose itself.
From this point of view, it is necessary that nitrocelluloses for use in polyurethane paints comprise nitrocellulose and a volatile component which does not react with isocyanate and does not remain in a coated film. For example, in accordance with U.S. Pat. Nos. 3,284,253 and 3,341,515, the wetting of nitrocellulose is carried out using hydrocarbons which do not dissolve nitrocellulose such as toluene. With the nitrocellulose so obtained, water is removed in the course of production, and thus it is considered to meet with the above described requirements.
The nitrocellulose thus-obtained, however, has not been in widespread use commercially because it has a drawback with respect to stability, i.e., the hydrocarbons used as wetting agents, which do not dissolve the nitrocellulose, are generally low in electric conductivity. Therefore, nitrocelluloses containing such hydrocarbons tend to become strongly electrostatically charged; furthermore, nitrocellulose particles are poor in flexibility, hard, and brittle, and are easily pulverized. These factors increase the possibility of the nitrocellulose catching fire by electrical discharge therefrom or the scattering thereof.